Instability after adding batteries...

Discussion in 'Metal Boat Building' started by cgoodwin, Dec 8, 2014.

  1. cgoodwin
    Joined: Mar 2011
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    cgoodwin Junior Member

    The vessel is a 1952 Feadship, 56' LOA, 13'2" Beam and a draft of 4'6". Steel hull, twin 671 detroits.

    I just finished installing 16 AGM batteries weighing 100lbs each. They were installed almost exactly in the center of the vessel lengthwise. They are just slightly to port of centerline and are about 20" below waterline.

    As soon as I installed the bank I noticed that the vessel has become "tippy", as I step onto the side deck from the dock she now noticeably rocks, which she had previously not done.

    I must say I'm a bit confused as I would think that adding 1600 lbs of weight below waterline would have made her more stable, not less.

    When I purchased the vessel she had 1200 lbs of batteries in nearly the same location although they were slightly starboard which served as starting and house bank, I now have 1600 lbs of batteries slightly port, very slightly in that same location with an additional 200lbs of batteries outboard of each motor. Overall I have added an additional 800 lbs of batteries and that additional weight is distributed widely.


    In her original (from the factory) configuration she had batteries outboard of the motors, like now, and the battery bank was about 600 lbs and mounted starboard of center.


    This is a 64 year old vessel and has undergone many changes. Originally there was a genset on the centerline aft, at some time this was moved all the way forward and as far port as possible in the engine bay. A 50 gallon (500lb full) waste tank was also added forward and starboard. When I purchased the vessel the entire foredeck was rusted under the teak and the steel had expanded from 1/4" to nearly 3" of wet rust. I can not begin to calculate how much weight i removed by cutting this out and replacing it, but replacing the entire deck did raise the waterline about 2" on a 56' long 13' wide vessel weighing 28 tons...


    Looking at it mechanically, adding weight low would move the center gravity closer to the fulcrum point by lowering the center of gravity, this would explain why the action of weight added to the outside deck (end of the lever) would more easily tip the vessel. At some point in the history of the vessel this was obviously an issue as bilge keels were added.

    In addition she has a 475 gallon midships fuel tank which has been empty since I began this project 6 years ago. This tank is forward of center.

    What confuses me is this: As I removed weight from the boat the bow came up about 6", the stern remained the same. The vessel remained stable. Adding the 1600 lbs of batteries just forward of center and below waterline did not bring the bow down or the stern noticeably, but did have the instant effect of making the vessel feel more "tippy". As I step on the side deck she tips and as other vessels pass the wakes now cause her to bob and tip more than before.

    I am finishing repairs on the midships tank this month and will be filling it and I imagine this will bring the bow down, but if adding weight below the water line made her more tippy, I am concerned that filling the tank will simply exasperate this issue.

    Any ideas?
     
  2. Mikeemc
    Joined: Nov 2014
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    Location: South Carolina

    Mikeemc Junior Member

    That boat may have dual tanks, check to see if the balance valve is open or to much fuel in one tank.
     
  3. cgoodwin
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    cgoodwin Junior Member

    It has tanks in the stern and the midships tank. Stern is full, midships empty, as they have been for 6 years. Still does not explain how adding batteries forward and below waterline makes the boat feel tippier....
     
  4. Mikeemc
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    Mikeemc Junior Member

    I'm familiar with that boat , we had an ASR with quite same dimensions. They are a little rollie , even at the dock. Burgers are the same , Hullmatics are the worst.

    Just a thought, when the tide was low the boat rocked more due to bottom wave currents. Have you moved to a new birth ?

    If the batteries are place fore of amidship in the narrow hull plane going forward to the bow it would seem that more weight is in the planning part of the hull ,pushing the center weight forward a bit causing it to bob on he forward plane when at rest. This is like a floating triangle with all the weight in the bottom point. Just my thought.
     
  5. cgoodwin
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    cgoodwin Junior Member

    That still does not explain why she would have been more stable before the addition of the batteries low in the bilge, one would think she would have become more stable, not less.
     
  6. Mikeemc
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    Mikeemc Junior Member

    I would agree with that statement on stability. I did do a little editing maybe there's a answer I'll have to give it more thought. Interesting.
     
  7. Mikeemc
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    Mikeemc Junior Member

    BTW , we thought we were going to be smart and change the rope locker and put 300ft of chain in instead , pops big mistake lol , picked the stern up about 4in. Was that ever funny. I had to pull it all out by myself in 95degree weather , talk about hot. 4ft out 2ft slide back in was that a hoot.
     
  8. daiquiri
    Joined: May 2004
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    Location: Italy (Garda Lake) and Croatia (Istria)

    daiquiri Engineering and Design

    If the hull of your boat is similar to this one: http://www.charterworld.com/news/wp-content/uploads/2014/10/Feadship_Serena.jpg , then I think the explanation iis related to the shape of her waterlines. In that photo, her waterplane area appears to be of a semi-displacement type, very narrow forward and wide around the transom.

    The waterplane area has a related mathematical quantity which is called the "moment of inertia" or "second moment of area" around the longitudinal axis, indicated with "Ix". I won't bother you how it is calculated (you can check it here, if you wish: http://en.wikipedia.org/wiki/Second_moment_of_area), the important thing to know is that the value of Ix is closely related to the transverse stability for small angles of heel. The bigger the Ix, the more stable is the hull (heels less under a given load).

    So, this is what I think has happened:
    Without the added weight of batteries, the boat has a CoG placed at some point from the transom. She sits down with the bow slightly upwards and has a certain stability, which you have judged as satisfying. Her waterline area has a form which is very wide aft and in the midship, and then narrows down towards the bow. Similar to an arrow, to make it more simple to visualize. Being wide all the way to the transom, it has a fairly good value of Ix.

    When you have added the weight of the batteries ahead of the initial CoG, you have shifted the CoG forward. As a result, the boat has trimmed down and has changed the shape of the waterplane area. It is now slightly more narrow towards the transom, as wide as before in the midhip area, and then narrows down again towards the bow. you can visualize it like a slightly more po├Čnty arrow which narrows noot only toowards the bow but also somewhat towards the transom too. This change causes a loss of some of the Ix of the waterplane, and this is the cause of the loss of the initial stability which you have felt.

    Cheers
     
  9. cgoodwin
    Joined: Mar 2011
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    cgoodwin Junior Member

    Thanks daiquiri,
    the odd thing is that the stern did not lift and the bow did not drop. Her attitude is bow light for sure, about 6" of bottom paint is showing below the boot stripe compared to 1" aft.
    Her hull is not really similar to the one pictured, the is quite narrow forward and the widest and deepest at the engine room midships with a round bottom. Aft she slopes from the engine room to the transom with the point where the transom meets the water very nearly level with the waters surface. She is a twin screw with a single rudder with the screws very close together.
     

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  10. cgoodwin
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    cgoodwin Junior Member

    Hull aft, you can see the bilge keels that were added at some point.
     

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  11. cgoodwin
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    cgoodwin Junior Member

    Hull forward
     

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  12. essenmein
    Joined: Dec 2014
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    essenmein Junior Member

    maybe the bats are above center of buoyancy of the hull, just being below the water line doesn't necessarily help with stability.
     
  13. cgoodwin
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    cgoodwin Junior Member

    Not really possible, the batteries are 7" above the bilge and 20" high or the center of this battery bank would be at 17" above the keel. She draws 4'3" (51") and the center of buoyancy is located at the center of gravity of the volume of the displaced liquid and would typically be above center of the draft, center being 25 1/2" above the keel. It is therefore impossible for the batteries in their current location to be above the center of buoyancy. Furthermore, the center of gravity is almost always above the center of buoyancy except in a keel sailboat.
     
  14. cgoodwin
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    cgoodwin Junior Member


  15. WindRaf
    Joined: Oct 2014
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    WindRaf Senior Member

    from what I read I understand that has changed the trim, especially the longitudinal, and so the center of gravity and of the hull no longer coincide as they should, that is, there are new centers and a new behavior of the boat.
    Also the addition of weight below the waterline increases the roll because it has pendulum effect when if is slightly transverse misaligned.
     
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